1. Field of the Invention
This invention relates generally to a communications system, and, more particularly to a method for predicting the probability of successfully recovering frames of (coded) information received over a communications link.
2. Description of the Related Art
In a typical wireless communications session, information is transmitted to and from a mobile device over one or more links or channels with the information being grouped into segments or frames. A frame is a basic timing interval in the system and at the same time, a frame refers to the sequence of contiguous bits that represents the user and control information that is transmitted during this basic timing interval. A frame typically consists of one or several slots, the latter being the channel's unit time interval. In some applications, the frame duration and the signaling rate may vary substantially, depending, among other things, on the channel conditions and the number of active users. A frame may therefore consist of a sequence of user and control information, the length of which is determined by the frame duration and the signaling rate. The quality of the received signals that constitutes the frame strongly depends on the distance between the mobile and the receiver, the terrain, and the activity of other active mobile users. A mobile device may for example suddenly move to a region where interference may quickly but temporarily degrade the quality of the communications link. Various types of error control codes and (hybrid) automatic repeat request (ARQ) or incremental redundancy (IR) protocols are being used to make the frames more resilient against channel noise and interference. In some applications the information is encoded with convolutional codes or Turbo codes to assist in error correction of the frame.
The changes in the quality of the link may mean that the receiver does not accurately receive some or all of the transmitted signals. Consequently, some or all of the information contained within one or more frames may be degraded and it may or may not be possible to recover the transmitted information with the error control mechanisms and (re)transmission strategies that are in use. Error control codes are also typically used to determine whether or not the decoded frame information is error-free. The frame error rate (FER) is defined as the percentage of frames that still contains errors after decoding. In some applications, this rate is typically around 1%.
It would be very useful to accurately predict the frame error rate for a given error control code and (re)transmission strategy when only limited information about the signal strength and the interference is available. Such a prediction method would have multiple applications: in system design and development to quickly evaluate error control mechanisms and scheduling, and in actual mobile communication systems for instantaneous monitoring of the link level performance and for adaptively selecting the appropriate error control code, the signal power, the modulation type and the (re)transmission strategy for given (varying) channel conditions.
Such a procedure is useful to instantaneously provide valuable information, particularly in situations where the actually decoding of the frame would introduce too much latency. If the channel conditions are changing significantly over a short period of time, the error control code and retransmission strategy can be adapted immediately as long as the latency is small. That is, upon discovering that the link error probability has changed, the mobile device may attempt to adjust to the new situation be choosing a different error control code and retransmission'strategy such as to achieve the target link error rate. Quick and accurate estimation of the link error probability is useful in developing a communication system that swiftly adjusts to the channel conditions and provides relatively clear and uninterrupted communications.
Quick and accurate estimation of the link error probability has proven somewhat problematic. To accurately determine that a link error, i.e., a frame decoding error, has occurred, substantial analyses must be performed on a suspected erroneous frame of information. The analyses typically involve decoding the frame to retrieve and check the information, which requires a significant amount of computing power and inherently results in delays. To ensure accuracy, these analyses can take considerable time, which may make the communications relatively unresponsive. For example, when transmitting voice signals, a significant lapse in the time between receiving the frame and detecting that it is in error may make it impossible to adapt the methods to improve the transmission in a timely fashion. On the other hand, if the analyses are faster but less rigorous, frames may well be considered uncorrectable when in fact they can be corrected. With less rigorous analyses, the link error probability will be quite unreliable and not useful for adaptive control algorithms.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.